1. Field of the Invention
The present invention relates to a sheet processing apparatus for performing post-processing on sheets having images formed thereon, a method of controlling the sheet processing apparatus, and an image forming apparatus equipped with the sheet processing apparatus.
2. Description of the Related Art
In recent years, an image forming apparatus for forming images on sheets has been generally equipped with a sheet processing apparatus called a finisher. The finisher aligns the edges of sheets discharged from the image forming apparatus, using a sheet alignment device, and then carries out post-processing including punching for punching holes in each of the sheets, stapling sheets stacked in a bundle form into one bundle, sheet sorting, etc. The sheet processing apparatus for performing such post-processing is demanded to execute the post-processing, such as pinching and stapling, with high accuracy so as to enhance the quality of products.
For example, in the case of punching, it is required to highly accurately position holes to be punched, so as to prevent dislocation of holes. However, a sheet conveyed into a sheet processing apparatus from an image forming apparatus can be laterally displaced in a direction orthogonal to a sheet conveying direction or skewed. In order to punch holes in such a sheet with high accuracy, the lateral displacement or skew of the sheet is corrected before punching.
For example, there has been proposed a device configured to punch holes in a sheet after correcting displacement of the sheet and then offset the sheet for sorting after completion of the punching (see e.g. Japanese Patent Laid-Open Publication No. 2003-226464).
Further, there has been proposed a device configured to punch holes in a sheet after correcting skew of the sheet by bringing the sheet into abutment with an abutment member (see US Patent Publication No. 2007/0029719).
The sheet processing apparatus is further demanded to perform post-processing with high accuracy without reducing productivity. In other words, the sheet processing apparatus is demanded to perform post-processing with high accuracy and in a short time.
However, the conventional sheet processing apparatus suffers from the following problem: In the conventional sheet processing apparatus, the skewed side of a sheet is brought into abutment with the abutment member, whereby the skew of the sheet side is corrected. Then, punching is performed on the sheet held in abutment with the abutment member, and the punched sheet is shifted laterally for sorting in the direction orthogonal to the sheet conveying direction. In this case, the sheet is moved in a state held in contact with the abutment member, and hence a frictional force is generated between the sheet and the abutment member. As a consequence, extra load due to the frictional force is applied to a motor for offsetting sheets. To withstand the increased load, the motor requires an increased size, which results in an increase in the manufacturing costs of the sheet processing apparatus.